1. Field of the Invention
The present invention relates to novel soluble polyimides, which have a high heat resistance and which are soluble in some organic solvents and capable of forming a film.
2. Description of the Related Art
It is already known in the art that polyimide type polymers have extremely excellent properties, for example, heat resistance and chemical resistance, and therefore, these polyimides are especially useful as wire coatings, films, and adhesives.
Polyimides are at present produced in the following manner. That is, polyamic acids are first prepared from tetracarboxylic acid components and diamine components, and the resultant polyamic acids are then converted to imides by various methods (i.e., the so-called two step production of polyimides). However, the above-mentioned intermediate polyamides have a poor storage stability and a high, unpreferable viscosity, causing geletion even at an ambient temperature, although these polyamides have an excellent heat resistance, and thus, they must be stored under a low temperature. Furthermore, when polyamides are allowed to stand for a long period of time, a portion thereof is dehydrated to cause the ring closure, whereby polyimides are formed and insolubilized to cause the formation of turbidity.
The above-mentioned disadvantages of the production and the processing of imide type polymers can be obviated by synthesizing solvent-soluble high molecular weight polyimides. For example, Japanese Examined Patent Publication (Kokoku) No. 47-37706 and Japanese Unexamined Patent Publication (Kokoku) No. 56-36520 disclose soluble polyimides produced by using, as a tetracarboxylic acid component or a diamine component, aliphatic compounds. However, the most important property of polyimides, the heat resistance, is unpreferably decreased in these polyimides.
Furthermore, polyimides, which are derived from biphenyl tetracarboxylic acid and p-phenylene diamine, disclosed in Japanese Unexamined Patent Publication (Kokai) No. 50-113597 can maintain a high heat resistance, but can be dissolved only in phenol solvents such as m-cresol and xylenol. However, it is difficult to dissolve polyimides at a high concentration with these solvents, and in addition, it is difficult to produce polyimide films having excellent mechanical properties. The cardo polymers are summarized in J. Macromol Chem., C 11 (1), 45-142 (1974).